The left atrium forms the base of the heart.
It has a smooth walled portion from the developmental incorporation of a portion of the pulmonary veins and an auricular appendage which
is ridged with pectinate muscles and which was the primitive left atrium.
Four pulmonary veins empty into the left atrium.
The interatrial septum is nearly transverse, sloping posteriorly and to the right
The left atrioventricular orifice leads to the left ventricle.Left Ventricle
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The left ventricle is nearly twice as thick as the right ventricle, since it performs more work to circulate blood to the body.
It has a conical cavity, the apex of which is the apex of the heart.
Its walls are lined with trabeculae carnea, which are more numerous and not as massive as the ones in the right ventricle.
It contains only two papillary muscles, anterior and posterior, which both attach to the two leaflets of the left atrioventricular, or mitral, valve via
chordae tendinea.
These muscles contract before the ventricle itself contracts, tightening the chordae tendinea and drawing the cusps of the valve together to
prevent backflow of blood into the left atrium.
The aortic orifice is located posteriorly and superiorly and, like the pulmonary orifice, is surrounded by a fibrous ring to which the three
cusps of the aortic valve are attached.
The aortic valve has three cusps, which open like pockets to catch blood that backflows from the aorta during ventricular diastole.
There are dilations in the wall of the aorta beneath each cusp: the aortic sinuses.
The right coronary artery originates in the right aortic sinus, while the left coronary artery originates in the left aortic sinus.Coronary Circulation
[Plate 211, Coronary Arteries and Cardiac Veins]